Heretofore a circuit indicated in FIG. 5 has been utilized as a reset circuit for a microcomputer controlling audio devices for use in a vehicle. In this figure C and R represent a condenser and a resistance, respectively, and they are connected with the power source line L for the microcomputer CPU. This circuit is so constructed that, when the voltage +B of the power source fluctuates, its variations are detected by this C and R and a reset pulse is given to the reset terminal of the CPU. The number of parts of this circuit is small and cost merit is obtained. However, in this circuit, the waveform of the reset signal is unstable due to the manner of the voltage rise in a 5 V line and to instantaneous interruption of the 5 V line, which gives rise to erroneous operations of the microcomputer due to the fact that eventually it is not reset or it is reset several times. Further the reset circuit itself detects only variations in voltage at the 5 V line and it plays no role in protecting the microcomputer against entering abnormal states (uncontrollable run due to unsufficient reset).
Recently microcomputers are utilized more and more in various devices. As the number of their complicated functions increases, maintaining memory contents becomes more and more important and a one-way reset circuit detecting only variations in voltage in the power supply line has proven unsatisfactory. Further, in such a prior art circuit, whether a reset signal was produced or not dependedon the time constant of an RC circuit. There were often inconveniences in that the microcomputer was not restarted for the reason that no reset signal was produced by the reset circuit in circumstances where it should have been produced and in that the memory was destroyed by a reset signal produced when the microcomputer was not stopped.